1. Field of the Invention
The present invention relates to a laser beam printer having a rotatable photosensitive member and rotatable deflection means capable of deflecting a laser beam toward the photosensitive member.
2. Description of the Related Art
Recently, optical beam printers have been widely used each of which is capable of forming a desired image by scanning a photosensitive member by an optical beam such as a laser beam which has been modulated by an image signal.
In a laser beam type printer of the optical beam printers, the laser beam is ordinarily deflected by rotating a rotational mirror such as a rotational polygonal mirror and a Galvano mirror or a hologram.
A laser beam printer of the type describe above is arranged in such a manner that the rotation of a photosensitive member is started simultaneously with the rotation start of a deflector. While waiting for the deflector to reach a predetermined rotational speed, pre-operational processing for the photosensitive member is performed. When the rotation of the deflector reaches a predetermined speed, the pre-operational processing is stopped and image forming commences.
The pre-operational processing will be described.
The sensitivity of a photosensitive member changes depending upon the duration of time between image forming operations.
Therefore, during pre-operational processing the photosensitive member is rotated prior to image forming, and, for the purpose of stabilizing the sensitivity of the photosensitive member, the exposure lamp and the primary charger are operated.
The rotation of the photosensitive member during pre-operational processing (hereinafter "pre-rotation") is performed in such a manner that the photosensitive member is rotated by one or more revolutions so that the sensitivity is stabilized over the entire surface of the photosensitive member.
Typically, the resolution, that is, the dot density realized by a laser beam is 300 DPI, the printing speed is 8 paper sheets per minute, the revolution speed of a rotational polygonal mirror having 6 planes at the time of the light deflection operation is about 5566 rpm, the diameter of the photosensitive member is 15 mm and the processing speed (the movement speed on the surface of the photosensitive member) is 15 .pi.mm (.pi.=3.14159). The time taken to make the driving motor for the rotational polygonal mirror rotate stably at 5566 rpm is 3 to 4 seconds. Furthermore, its sequence is arranged in such a manner that the rotation of the driving motor is started simultaneously with the start of the photosensitive drum in response to the turning on of the print signal. In consideration of the life of the photosensitive member, pre-rotation is arranged to be performed by two revolutions, that is two seconds. Therefore, by the time the rotation of the polygonal-mirror rotating motor reaches the predetermined speed, pre-rotation of the photosensitive drum has been completed. As a result, constant speed rotation can be realized and image forming can begin.
Recently, a scanning optical device employing a small-diameter polygonal mirror has been developed so as to meet the desire to reduce the size of the apparatus. Therefore, the diameter of the rotor of the polygonal-mirror rotating motor was also reduced, causing the inertia moment of the rotating portions to be smaller. As a result, the time taken to reach constant rotating speed is one second or shorter. Therefore, the above-described pre-rotation sequence cannot be completed by the time the mirror reaches its predetermined speed, and image quality decreases.
Pre-rotation must, usually, be performed by two revolutions or more. Therefore, if the above-described pre-rotation cannot be performed, pre-operational processing cannot be performed sufficiently, causing the sensitivity of the photosensitive member to become non-uniform.
Although a structure has been disclosed in order to overcome the above-described problem, the structure capable of controlling the starting time must be provided with a complicated control circuit, causing the overall cost to be raised excessively. It has been considered to employ another structure arranged in such a manner that the rotation of the polygonal-mirror rotating motor is continued until the completion of the pre-rotation. However, the useless rotation of the polygonal-mirror rotating motor may use energy wastefully, and what is even worse, the life of the polygonal-mirror rotating motor is undesirably shortened. In addition, an excessively large electric current passes if the polygonal-mirror rotating motor and the drum driving motor are simultaneously started. Therefore, a problem arises in that a large power source must be provided.